Water in oil emulsion drilling fluids always have drawbacks such as low yield point, low ratio of yield point to plastic viscosity, and poor suspension property, etc., and can't effectively carry rock cuttings and suspend the weight materials contained in them owing to these drawbacks. In the actual drilling process, lowing shearing force may cause solid sedimentation during circulation of the water in oil emulsion drilling fluid, reduced bit penetration, increased solid concentration in the drilling fluid at the bottom of well, and thereby result in complex downhole conditions such as balled-up bit, etc. Water in oil emulsion drilling fluids usually employ organic clay as a viscosity and shearing strength improving agent and rely on the interaction between the organic clay and the water droplets in the invert oil emulsion to form a spatial grid structure to improve the yield point of the drilling fluid system. Different from the bentonite used in water-based drilling fluids, the organic clay has a low gel rate, and the surfactant contained in the organic clay may fall off at a high temperature and result in organic clay thickening and failure. As the well depth and the downhole temperature increase, in conjunction with the development of low-clay or clay-free high-performance water in oil emulsion drilling fluids, the requirement for shearing strength and suspension property of drilling fluids can't be met merely with organic clay.
At present, there are only a few types of shearing strength improving agents for water in oil emulsion drilling fluids in the world. These shearing strength improving agents for water in oil emulsion drilling fluids can be categorized into organic low molecular ones, rubber-based ones, and amphipathic molecular ones. Organic low molecular shearing strength improving agents mainly include low molecular alcohols, ketones, esters, and their mixtures, such as ethyl glycol, glycerol, carbonic allyl ester, and butyl acrylate, etc. Rubber-based shearing strength improving agents mainly include styr-butadiene rubber, ethyl-propyl rubber, and vinyl thermoplastic elastomers. Amphipathic molecular shearing strength improving agents are mainly chemical compounds that employ amide groups or ester groups as the polar groups and long chain alkyl groups as non-polar groups. However, these shearing strength improving agents have their drawbacks, mainly represented in: organic low molecular shearing strength improving agents excessively rely on an organic clay environment, can't take effect in drilling fluids without organic clay, and may have impacts on the stability of the water-in-oil emulsion; in addition, ketone or ester compounds have side effects on the environment; rubber-based shearing strength improving agents have a low shearing strength and are difficult to dissolve, and thereby are difficult to use in the field; amphipathic molecular shearing strength improving agents have a poor shearing strength improvement effect and may greatly increase the plastic viscosity, and thereby are adverse to the improvement of ratio of yield point to plastic viscosity of the water in oil emulsion drilling fluid.